Caption: Avian flu virus, computer illustration. At the core of the virus is RNA (ribonucleic acid) genetic material. This is surrounded by a nucleocapsid and a lipid envelope. The coat contains surface proteins (spikes). Flu viruses have two types of surface protein, called haemagglutinin and neuraminidase. These exist in several subtypes, only some of which are pathogenic in humans, including the current bird flu strain H5N1. The surface proteins are essential for the life cycle of the virus. Haemagglutinin allows the virus to bind to and enter a host cell, where the virus uses the cell's machinery to create more copies of the virus. Neuraminidase allows the new virus particles to exit the cell, so that they can infect others.